Method of extrusion



Jan. 21, 195s R. BKQCH METHOD oF Ex'rRusIoN Filed oc. 18, 1954 AM A ORNEYS METHOD F EXTRUSION Robert B. Koch, Hyde Park, Pa., assignor'to The Polymer Corporation, Reading, Pa., a corporation of Pennsylvania Application October 18, 1954, Serial No. 462,647

7 Claims. (Cl. 18-55) This invention relates to extrusion of thermo-plastic materials to form elongated shapes, the invention being concerned with an extrusion method and a die structure especially adapted to the continuous extrusion of elongated shapes from synthetic linear polyamides, more particularly the so-called high melting polyamides, such as polyhexamethylene adipamide, polyhexamethylene sebacamide and polyepsilon aminocaproic acid.

Among the principal objects of the invention are the irnprovement of dimensional accuracy and surface-smoothness of the shapes extruded, while at the same time minimizing wear of the extrusion die.

How the foregoing general objects are attained is set forth more fully hereinafter following a description of a preferred embodiment of the die structure and of extrusion equipment arranged to provide for the practice of the method of the invention, all of which features are illustrated in the accompanying drawings, in which- Figure l is a plan view of equipment arranged to provide for the extrusion of tubing according to the invention; and

Figure 2 is an enlarged side elevational view of the equipment of Figure l, with certain portions broken out and others shown in vertical section for simplicity and clarity of illustration.

The barrel of an extruder or screw feed device is illustrated in outline at 3. This device may be of known construction, in which a screw feed device operates in the barrel, the polyamide material being fed to the device and advanced by the screw for extrusion. During advance of the material through the barrel heat is applied thereto so as to melt the polyamide, and the melted, i. e., liquid polyamide is delivered from the barrel through a passage 4 into the die structure generally indicated at 5.

The die structure comprises an assembly of parts 6, 7 and 8 having a central chamber 9 therein with which the passage 4 communicates so as to receive the liquid polyamide from the extruder. Toward the delivery end of the die structure a nozzle part 10 is arranged having a cavity therein in the form of a tapering continuation of the cavity 9 and adjacent the delivery face of the die structure the extrusion die itself is positioned, as clearly appears at 11 in Figure 2. This die 11 may also have a somewhat tapered passage therethrough which constitutes the extrusion passage itself. Centrally arranged within the die structure is an adjustable pin 12 having threaded connection with the surrounding sleeve 13, by means of which the pin may be advanced or retracted with reference to the die opening. The pin 12 has a central passage formed therethrough to provide for the admission of air during the extrusion of the tubing from the die passage. A retainer piece 14 of washer-like form is positioned at the exit face of the die 11 and is centrally cut away or beveled as indicated at 15 in the region where the extruded tubing leaves the die 11. Therefore, the extrusion of the formed tubing occurs from the aperture in the die 11, without contact with the washer 14.

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As clearly appears in the drawings, the extrusion die structure is arranged immediately at one end of an elongated vessel or trough 16 having side, bottom and end walls, but being upwardly open, and serving to contain the liquid of a bath into which the tubing is directly extruded from the die structure. For this purpose the adjacent end wall 17 of the trough is apertured as indicated at 18, this aperture being located below the level L of the bath, as appears from Figure 2. In this way the liquid of the bath has direct access to at least that portion of the surface of the extrusion die 11 imf mediately adjacent the extrusion aperture.

The extruded tubing is drawn lengthwise through and then out of the bath in the trough 16 by means of `a pair of take-off rolls 19, from which the tubing passes to a take-up reel 20, on which it is wound. The take-off rolls are provided with drive mechanism (not shown) by which the speed of drawing of the tubing through the bath is determined, the drive for the reel 20 incorporating a slip means so that it merely serves to wind up the tubing fed to it from the take-off rolls.

The liquid of the bath may be circulated through a heat exchange device adapted to provide for temperature control, as by means of circulation connections l121--21 shown in Figure 2. With regard to the illustration of the bath and trough in Figure 2 it will be noted that the mid portion thereof is broken out, the circulation connections 21-21 preferably being located relatively closetothe ends of the trough. .t

In the practice of the invention, two importantl features are preferably utilized. One of these relates to the composition of the material of which the extrusiondie llis formed, and the other relates to the composition ofthe bath into which the tubing or other elongated shape is directly extruded from the die 11. These two features or factors mutually contribute to the two important objectives hereinabove referred to, i. e., the improvement lof dimensional accuracy and surface smoothness of the extruded shape, while at the same time minimizing die wear.

The character of the die itself is here first considered. It is known that polytetrailuoroethylene has surface characteristics which are advantageous when this material is employed as an extrusion die. This material, however, tends to wear relatively rapidly and in addition tends to wear irregularly, thereby impairing the desired dimensions and surface smoothness of the shape being extruded.

According to the present invention the die is formed of polytetrauoroethylene filled with tine silica particles. I have found that silicay is strikingly distinguished from other theoretically usable fillers, in that the silicajcontributes a high degree of hardness and wear resistance without requiring excessively high loading, so that the hardness and wear resistance are provided without destroying the desirable surface characteristic contributed by the polytetrauoroethylene. In preparation of the die, from about 5% to about 65% of silica maybe used, preferably in the range from about 15% to 40 or 50% where the die is to be employed in the extrusion of the high melting polyamides.

The silica employed is preferably of the highly porous type and of fine particle size, say from .5 to 60 microns, a preferred particle size averaging in the range from l to 20 microns. In typical porous silica, the pore size is of the order of 20 to 100 Angstrom units.

The following description illustrates the preparation of a typical die piece of about grams weight.

g. of 50% polytetrauoroethylene aqueoussuspensoid (containing some wetting or dispersing agent and ammonia) are used, and to this 420 g. of Water are added, with stirring. 30 g. of silica of particle Size averaging from 3 to 7 microns are also added stir'.

ring, after which.,glass mill Yballs areY iutroducedand-.the mixture is tumbled until a dough-like mass precipitates (about 6 to 24 hours).

- Ther-precipitated dough-like mass '-is =then`dried Fin an voven 'at-a temperature--ibetweeni aboutv 550 FF. fand/162W v151, so as f to driveftfwettingsagentfiaridealso :waterwemainin-g. The resvltant-'driedfmassr-is then broken-iup, --forming featherypieceslV andthisfmaterial isipaclcedin a forming die* and-=1subjeeted toiheating Hat-'from 5627.10 750i-F.,underia1pressure-of" 5,000- to '11;000713. s. -i. `'-1he-forming die, preferably conforms with `lthe .Shaper of the peceftofbeimade; 'and `to ,complete ftherdiefthecentral dieaperture` is 'cut -thereinjfor -instanceaof 'the shape illustrated inthe drawings.

The foregoing :procedure A'provides Yafdie. piece containing labout l 230% of silica* by -weight and.y having exj-ceptional hardness andi-"wear .resistance LA die .piece formed Y according to fthe Einvention isalso` characterized thy-unusually low loss of hardness uponelevation :ini-temperature, even up to aftemperatureof the'forder-'of 575 The hardnessand low loss are illustrated-'in' comparison with-unfilled polytetrauoroethylene in the following vtable .giving- Shore durometerA hardness -readings 4on the D-scale.

The""extrusion "die' ofthe inventon'v'retains Yatdeast some of the desirable surface characteristics of' the polyftetrauoroethyleneand rfurther has a 'thermal'conduc- "tivity comparable to that of the unfilled' polytetrauoroethylene.

"The'debffthe inventionis also characterizedby a 'much -lov`ver"coetlicient of thermalexpansion"than"a 'die "formed of unfilled polytetratluoroethylene. Indeed, fvvithabout 50% `silica the 'coefficientevenapproaches ffthatfof steel.

The'fforegoinglfconjparative I"results are" based vupon aetu'aloperations in whichlthe tubing was drectlyex- -trudedlin`to a"water'bath. -eAsiW'illbelpoirited out'more ifullyihereinfter; fwhenemployingtne `silica-inked4v ldie `t0- -'gether Withabath of the composition contemplated by -thepresentnvention,'ithe cwear on the silica-filledv die 'ist-even'` further reduced.

ilBeforelconsidering`thelcompositionot lthe bath, it may rst be mentioned that in"the"'eXtrusion of'tubing frotn'i a. Ahigh fm'elti'ng polyamide, the liquid" polyamide yenters "the""c`entral cavity of "the die structure under the pess'rei'exertedby the' screw working in'the 'ba'r'rel 3. EThefliqnid condition "of "thepolyamide'is' maintained during its `passa'gethron'gh''-the die structu're, for which pnrpose'heater elements'.'(not'lshown) are associated with themain'tbodyf'thedie structure. "Theftemperature of ihetig'will, ofcoirseffdependupon thefparticular mavt'ial'being" 'etrudedjatidir 1the 'case' bffthe liighmelt- "ting-p lyaniides will be'V` above"`the s'olidificationtpoint nner "ffs. instanceftntheaaage -fromabont '10 r. to "173 :FI- "abovef'the"'solidcationpoint As the' material A`is.l aboutmto .leavetthedie 11, -i. e., f in the closel vicinity of the exit face of the die, the cooling action of the bath initiates soliditication, and for this purpose the temperature of the bath is advantageously kept below F., most desirably below about 60 F.

To reduce excessivechillingof the die itself by contact with the bath, an insulating washer such as indicated at 14 may belused. 'Infatypicalcaseisuch an insulating washer may comprise aglass nbre laminate lledwith polytetraiiuoroethylene However, ."itis oflimportance Vthat the extrusion actually'take place from the-die 11 directly into the bath, for reasons which will further appear Lhereinafter in` :the vfollowing4 discussion of A:the composition of the bathyand wbecause of this there should be some exposure of the die tothe bath.

i have found that the introduction of a wetting agent into the water of the setting bath definitely improves the v@extrusionl operation; and Ystill further that the introduction 1 oa material havingboth='wetting and1lubric'ating lcharaclfteri'sti'cs effects additional improvement yas compared with vlai-'material having wetting vaction only.

fIn the firstV place, y'the-'presence ofanL agenti having'a `--wetting la'ction improvesi the heat*transferbetween the rJsurfa'ce of-'fthefsolidifying shape and "thesettingbath,

Boiling and bumping ofthe'bath onlthessurface of lthe extruded shape is -virtually eliminatedlfand this Aimproves "-"thesurfa'ce smoothness anddimensional 'accuracyof the -v"extruded shape.

Where the material added*to"'theba`th has bothtwetting 'and' lubricating characteristics a 'still further striking' irn- `*prot/"ement in operation is effected. VThis `relates tothe v1lwearoccurring 4in theextrusionpassage' ofthe die itself. Wherethe material'added" to 'the bath has both of the characteristics mentioned the'wear :is extensively reduced beyond 'the'-reductionobtained merely by the employment ff thef silica as yfiller in the polytetrauoroethylene die.

While all Vthe' reasons for thisv may not be completely understood, the extrusion of the piece from thedie direct- "'ly into the bath containing the wetting and lubricating vagent, 'andthe contact of the die with thewe'tting and l'ubricating 'agent 2favorably influences 'the smoothness with which the actual extrusion occurs. l2It" is"beli'eved "that someof the-agent isactually4 abso`rbed"'intov the exit "face ofthe die.

Ina `test'-run"of fthe-'kind :referred to above where "I'V. D.'polyh'e'xam'ethylene adipamide 'tubing was being --e ttruded,-z when lthe setting bath contained condensation `-p'roductgas'a' wetting agent Triton X-lOO, an'alkyl phenolvethylene'"oxide alcohol`marketed bye Rohm" & Haas;v the lrweafonr#30% silicaf-lled die after a 12-hour`run'was 'not`- even measurable.

.'Wi-threspect to `the A'lubricating'- 'action' 'Withinthe I die, *itis pointed "out" that where' the VAdie 'contains upwards'of about i36% -l'silicajirregular `or pulsating extrusion :will occur *'(as 1a 'result"of"sticking of the polyamide in'the "dief" if tsachliche @is employed for "extrusion 'into water 4alone. z'5'I-ovveverf'I*have fot1nd"`that" even with silica loadingswell above'3il%,'l for instance up"to"50 or`60%, rthister'adency forthev polyamide "to"s'ticli.in"the"die can be Icompletelyo"ve'rcon1eby introduction pfapplipriate ma- "t'erials'linto klfthe" settingba'th. v'fTh'us; even 'at very high l'silica' loadings,"v the introduction of an agent (or a. comyposite ef agents) 'having both wetting' andlubricating'char- Ja'cteristics,- entirely Smooth' operation 'can' be` achieved.

'Itis alsopointed out thatre'gardless of .the percentage "loading of"silica,'the presence o'an agent in" thelsetting bath havinglbth lubricating and"w'etting characteristics 'achieves avsurfacesnroothnessofstrikingg almost mirror- 'like, 4inish.

*A'varie'ty of different[agentsy having either wetting aetion'alone. korhaving both wetting and lubricating .propertiesmaybe used in `the practice `.of lthe' invention. 'The 'commercially known" wetting lagents. generallynnay be employed" for the purpose of providing the desired wetting action. For wetting purposes it is preferred to employ either an anionic or a nonionic wetting agent. To achieve maximum improvement in the extrusion operation it is preferred to select an agent having not only wetting characteristics but also lubricating properties.

Specific agents usable for various of the purposes referred to are:

Aerosol OT-a sodium dioctyl sulfosuccinate made by American Cyanimid Co.

Tergitol 7-a sodium heptadecyl sulfonate and dichloro ethyl ether, made by Carbide and Carbon Chemical Co.

Vel-A sulfonated monoglyceride of coconut oil fatty acids, made by Colgate Palmolive Peet Co.

-An alkyl aryl sulfonate, made by Detergents, Inc.

Alli-a mixture of alkyl aryl polyether alcohol and a sodium salt of a sulfonated petroleum hydrocarbon, made by Lincoln Chemical Company.

Ultrawet 35 KX-a sodium salt of sulfonated petroleum hydrocarbon (C16) made by Atlantic Refining Co.

Cerfak N-25-an alkolyl amine made by E. F.

Houghton Co.

Triton NE-an alkyl phenol-ethylene oxide condensation product, made by Rohm & Haas.

Alkyl aryl polyether alcohols.

Of those mentioned above Ultrawet 35 KX, Triton NE and the alkyl aryl polyether alcohols have not only wetting properties but also excellent lubricating properties.

All of the others in the above list except for Aerosol OT and Tergitol 7 have some lubricating eect, but not equal to that of the three just mentioned.

In selection of the agent to be employed several factors should be kept in mind. First, the agent should preferably have a relatively high boiling and/ or decomposition point, so that, upon absorption into the extrusion die, it will remain in the desired physical and chemical state. With this in mind, the agent used should preferably have both boiling and decomposition points above 400 F., and preferably above 500 F. The agent selected should also be substantially inert with reference to the polyamides, even at temperatures corresponding to the melting point thereof. This is important in order to avoid any chemical or solvent action on the shape being extruded. The agent selected must also be water soluble, so that it can be incorporated into the setting bath.

The preferred concentration of the wetting or wetting and lubricating agent used in the setting bath will vary according to the particular agent selected. In general, even fractional percentages of such agents will effect at least some improvement. Indeed, in the case of certain agents, percentages running down as far as .01 or .02% have been found to produce a noticeable improve-- ment in the extrusion operation. Ordinarily, not more than about 3% is needed to produce an extensive eect, although additional quantities may be present with at least some of the agents. Preferably the agent is not present in quantities exceeding about 5 to 10%, particularly with certain individual agents, the effect of which has been found to diminish in concentrations upwards of about 5%. The presence of an excessively large quantity of various of the wetting agents has a tendency to reduce rather than increase the eiciency of heat transfer, and this tends to cause irregular or pulsating extrusion, and is therefore to be avoided.

As indicated above, because of the specific characteristics thereof, certain ot the wetting agents provide a wetting action only, without contributing any appreciable lubricating action. Agents in this class, for instance,

Aerosol OT, will serve to improve heat transfer from the extruded shape to the setting bath, because of the wetting action thereof, but will not, per se, reduce die wear or prevent sti-cking of the extruding material in the die. However, I have found that wetting agents of this type may be employed in combination with some other agent contributing the desired lubrication. For example, Safco #770, a petroleum oil emulsied with sulfonated soaps made by Swan Finch Oil Company, may be used in combination with Aerosol OT, in which event die wear is reduced and sticking is avoided, even where the extrusion die contains quantities of silica well above 30%. The Safco oil itself will not effect any appreciable improvement with respect to die wear and/ or sticking of the material in the die; and in view of this, it may be that the concurrent use of the wetting agent serves not only the purpose of improving heat transfer but also that further function of enabling absorption of some quantity of the oil into the extrusion die, and thereby provide the lubricating action required to avoid sticking and provide smooth extrusion, as well as reduction in die wear. An effective combination of these two agents is provided by inclusion of about .03% of Aerosol OT and .05% of the Safco oil.

I claim:

l. A method for forming elongated shapes from polyamides, which method comprises forcing molten polyamide through a die formed of polytetrauoroethylene filled with silica directly into a water bath containing a wetting agent.

2. A method according to claim l in which the silicai filled polytetrauoroethylene die comprises from 5% to 65% of silica.

3. A method according to claim l in which the water bath is maintained at a temperature below F.

4. A method according to claim l in which the water bath contains from .01% to 3% of the wetting agent.

5. A method for forming elongated shapes from polyamides, which method comprises forcing molten polyamide through a die formed of polytetraluoroethylene filled with silica directly into a water bath containing a water soluble oily liquid wetting agent.

6. A method for forming elongated shapes from polyamides, which method comprises forcing molten polyamide through a die formed of polytetrauoroethylene with silica particles dispersed therein, the silica particles comprising from 30% to 65 of the material of the die, the elongated shape being delivered from the die directly in a water bath containing an agent having wetting and lubricating characteristics. said agent being absorbable by the material of the die.

7. A method for forming elongated shapes from polyamides, which method comprises forcing molten polyamide through a die formed of polytetrauoroethylene with silica particles dispersed therein, the silica particles comprising from 30% to 65 of the material of the die, the elongated shape being delivered from the die directly in a water bath including at least one agent for imparting wettingand lubricating characteristics, at least said lubricating agent being absorbable by the material of the die.

References Cited in the tile of this patent UNITED STATES PATENTS 2,324,397 Hull July 13. 1943 2,403,476 Berry et al. July 9, 1946 

1. A METHOD FOR FORMING ELONGATED SHAPES FROM POLYAMIDES, WHICH METHOD COMPRISES FORCING MOLTEN POLYAMIDE THROUGH A DIE FORMED OF POLYTETRAFLUOROETHYLENE FILLED WITH SILICA DIRECTLY INTO A WATER BATH CONTAINING A WETTING AGENT. 